When a fuse blows, an arc is developed which, if it spreads to the metal surfaces of the fuse terminals, will vaporize the surface layer thereof and create fuse exploding pressures. In an AC circuit, the arc generally becomes extinguished as the AC current drops to zero and may not restrike or cause rupture of the fuse if the pressures and temperatures in the fuse cavity can be held within acceptable limits. As fuse structures are made progessively smaller, it becomes more difficult to keep these parameters within desired limits.
There is a need in the printed circuit art for fuses of substantial voltage rating, i.e. from 60 to 250 volts, and characterized by as small an overall dimension as possible. Such requirements are inherently in conflict, since a blowing fuse tends to generate rupture forces as a result of gas evolution and heating during the traveling of the arc along the fuse wire path and hence fuses capable of withstanding substantial restrike voltages during blowout typically must be fashioned with length greater than otherwise desired to allow the arc to extinguish and prevent rupture of the fuse casing. Should the casing rupture, there is an attendant fire hazard, as well as an attendant danger of damage to components on the printed circuit board itself. Printed circuit fuses should also have adequate protection against the entry of spray or dip solvents commonly used in the cleaning of printed circuit boards after final assembly of the components thereon.
To the applicant's knowledge, prior to the present invention there has not been designed a reliable sealed fuse much smaller than previous designs and capable of withstanding high energy fuse blowing conditions without destruction of the fuse housing. For example, there is a need for a reliable miniature printed circuit fuse which for a steady blowout current of 50 amps and 250 volts or equivalent energies can be made reliably as small as about 0.4 inches or less in overall length and even less in height and with a terminal spacing of the same dimension if desired (as when the terminals project axially from the fuse body ends and then bend downwardly). There has heretofore been developed cylindrical fuses with depending terminals within the boundries of the fuse and having a diameter of about 0.3 to 0.4 inches. The width of the fuses thus had to be greater than the terminal spacing and the height of the fuse was equal or greater than its width. Thus, at present, printed circuit fuses capable of withstanding such energies are relatively large, bulky fuses with cylindrical insulating bodies. Also such cylindrical fuses are too bulky for mounting on carrier strips wound on dispensing reels which can be conveniently inserted into automated machinery which automatically insert the fuses into the printed circuit board.
Other fuses used on printed circuit boards have fuse terminals which project from opposite axial ends of the body and terminate in parallel confronting terminal ends pluggable into socket openings in the printed circuit board, but these fuses when designed to accommodate the energies involved, have also been undesirably large. Since the general objective in printed circuitry is miniaturization, it is desirable that the fuse itself occupy as little space on the printed circuit board as possible.
It is frequently required of some low amperage fuses that they use fuse wire of very small diameter, such as the order of 0.0003 inches, for example. There is an inherent difficulty in fabricating fuses using such delicate fuse wires since the tensioning and positioning of such elements during delicate soldering operations is typically a manual operation resulting in substantial labor costs. Thus, an adequately miniaturized high voltage fuse of relatively low blowout current which could be manufactured inexpensively by automated methods would be a useful contribution to the art.
The prior art has used various techniques to increase the operating voltage of fuses by incorporating various techniques to increase the operating voltage of fuses by incorporating various arc quenching means therein. Thus, fuse elements have been surrounded by a suitable arc-quenching material. However, this approach is difficult to achieve in miniature fuses, or where very delicate fuse elements are used in the fuse. Another arc-quenching technique is to pass the portions of the fuse element immediately in advance of the points where they are soldered to the fuse terminals through restricted openings or grooves in the insulating material of the body involved, as shown by the fuse construction of U.S. Pat. No. 4,267,543, granted to Arikawa. This patent discloses a fuse structure employing a fuse element spanning a cavity defined between D-shaped insulating arc barrier-forming bosses in a cylindrical base portion of the fuse. The bosses are slotted to receive the fuse element and have recesses to receive and expose the terminals of the fuses to which the fuse element ends are soldered. A rigid cover overlies the base portion of the fuse. However, it is believed that the fuse design is inadequate to withstand without rupturing the pressures and temperatures present in a 250 volt circuit when made with less than 0.4 inch exposed to arcing terminal separation. Furthermore, because the circuit plug in terminals are spaced parallel pins, the overall size of such a fuse would be much greater than the terminal spacing.